Wind-tunnel balances are multi-dimensional force transducers used to obtain high-precision measurements of the aerodynamic loads on an aircraft model during wind-tunnel testing. In many wind tunnels, these aerodynamic measurements are made with the measurement device installed inside of the aircraft model, also known as an internal balance. Internal balances are electro-mechanical devices designed to isolate the aerodynamic load components on to a series of structural springs, or flexural elements. Strain-gage bridges are commonly used to measure strain induced in the flexure as a result of deflection. In theory, the flexure deflections and the resulting induced strain are proportional to the imparted load. Six-component internal balances are mechanically designed and instrumented with strain-gage bridges in one of the three following configurations:                Direct-Read (NF, PM, SF, YM, RM, AF)        Force (N1, N2, S1, S2, RM, AF)        Moment (PM1, PM2, YM1, YM2, RM, AF)        
The three types of internal balances measure rolling moment and axial force directly; however, the remaining four aerodynamic components are either measured directly, as in the case of direct-read, or are computed by combining sets of strain-gage bridges (i.e. for a force balance, NF=N1+N2 and PM=N1−N2). The type of internal balance impacts both the calibration and its use.
With reference to FIG. 1, a known internal force balance 20 has two attachment points. The non-metric end 22 of the balance 20 is grounded to the wind-tunnel model support system or model sting 38. The non-metric end 22 of the balance 20 does not contribute to the sensed force by the force balance 20. On the opposite end of the balance 20 is the metric end 24, which attaches to the wind-tunnel aircraft model 25. The metric end 24 is where the aerodynamic loads are transferred into the main body of the balance 20 and the flexure beams. In addition to the force balance 20, an angle measurement system (AMS) may be installed inside of the aircraft model 25 for measuring model orientation. Together, the aircraft model 25, model sting, force balance 20, AMS, and other instrumentation comprise a wind-tunnel model system (WTMS). Traditionally, these components are considered to be independent systems with little to no interaction effects.
A known approach involves characterizing the measurement systems independently and then the individual components are integrated prior to a test. Under the assumption of no interaction between the components of the WTMS, this approach is sufficient. It is known that interactions can exist between the components and therefore a methodology to understand these effects would be beneficial. A known method for assessing balance performance or WTMS interactions is known as check-loading. Check-loading includes applying a precision load using free-hanging deadweights, hydraulics, or a pneumatic system. Generally speaking, check-loading is currently used prior to a wind-tunnel test to ensure proper installation and integration of the WTMS components, and to validate that the balance is performing as expected. The latter requires knowledge of the calibration of a balance. As the primary source for aerodynamic force and moment data, balance calibrations play a critical role in the quality of the data collected during a wind-tunnel test; yet, the performance of a balance cannot be characterized in-situ while fully integrated in the WTMS. Furthermore, since there are few recommendations available, the process of check-loading may vary from wind tunnel to wind tunnel. This variability between facilities in combination with limited resources, such as traceable standards and required hardware, previously prohibited a robust system-level, in-situ characterization or validation of the WTMS.
Although the process of check-loading is not standardized, many facilities use simple mechanical hardware, similar to what is used in conventional calibrations, to apply a single- or two-component load to the balance or WTMS. More complex loadings are difficult to set-up and execute in a wind tunnel and may introduce more uncertainty than they quantify. Axial force check-loads may be difficult to apply because applying these loads requires a pulley and cable setup. This type of setup increases the odds that additional uncertainties will be introduced. In aeronautics research, axial force check-loads provide valuable information concerning the performance of a balance. However, these loads cannot be applied consistently and confidently utilizing conventional methods.
The AIAA paper “Recommended Practice: Calibration and Use of Internal Strain Gage Balances with Application to Wind Tunnel Testing,” Tech. Rep. R-091-2003, AIAA, 2003, briefly discusses the role of check-loads. The document suggests that check-loads are used to verify the scale factor of the gage bridges between two different environments, such as the calibration laboratory versus the wind-tunnel test section. Furthermore, it recommends that check-loads should be applied at the facility using the same hardware used during the calibration. The document does not address any standard procedures to be used during 9 balance check-loading, or any standard metrics that shall be used by the test engineer or researcher to evaluate the overall performance of the balance in the testing environment.